Pumping stations in sewage collection systems, also called lift stations, are normally designed to handle raw sewage that is fed from underground gravity pipelines (pipes that are laid at an angle so that a liquid can flow in one direction under gravity). Sewage is fed into and stored in an underground pit, commonly known as a wet well. The well is equipped with electrical instrumentation to detect the level of sewage present. When the sewage level rises to a predetermined point, a pump will be started to lift the sewage upward through a pressurized pipe system called a sewer force main from where the sewage is discharged into a gravity manhole. From here the cycle starts all over again until the sewage reaches its point of destination—usually a treatment plant. By this method, pumping stations are used to move waste to higher elevations. In the case of high sewage flows into the well (for example during peak flow periods and wet weather) additional pumps will be used. If this is insufficient, or in the case of failure of the pumping station, a backup in the sewer system can occur, leading to a sanitary sewer overflow and the discharge of raw sewage into the environment.
Sewage pumping stations are typically designed so that one pump or one set of pumps will handle normal peak flow conditions. Redundancy is built into the system so that in the event that any one pump is out of service, the remaining pump or pumps will handle the designed flow. In these days there are a lot of electronic controllers in the market designed specially for this application. The storage volume of the wet well between the ‘pump on’ and ‘pump off’ settings is designed to minimize pump starts and stops, but not enough to enable the pump to run continuously.
Traditional sewage pump stations incorporate both a wet well and a dry well. More modern sewage pumping stations do not require a dry well or pump house and usually comprise only a wet well. In this configuration, submersible sewage pumps with closely coupled electric motors are mounted within the wet well itself, submerged within the sewage. Due to the much reduced health and safety concerns, and smaller footprint and visibility, submersible pump sewage pumping stations have almost completely superseded traditional drywell sewage pumping stations.
Control panels provide the electrical power required to operate submersible pumps in lift stations. The traditional waste water pump station design requires five float switches including two sets of starts and stops for the primary pump (Pump-A) and the second pump (Pump-B) with the fifth float switch controlling the emergency alarm. Start switches for Pump-A and Pump-B are located at the wet well mid-elevation and are about six inches apart from each other. The lower start switch turns on Pump-A and the higher start switch turns on Pump-B. Both stop switches of Pump-A and Pump-B are located about six inches lower than the top of the motor and stop both pumps at the same time. A logic controller handles typical float switch and pump failures and can continue to operate the lift station on only one functioning float switch.
Efforts to improve energy efficiency of pump stations and to reduce maintenance costs of the pumps have been made over the years. The continual rise of energy costs to power the pumps has strained municipal and state budgets at a time when revenues have fallen. Infrastructure maintenance competes for tax dollars and is often put off as budget cutting is forced. The most highly touted improvement was the introduction of variable speed pumps in pump station design. The variable speed pumps were proven to reduce energy costs in other applications. However, in studies done by Dr. Thomas Walski, P. E comparing variable speed pumping to constant speed pumps in systems for the relatively flat system head curves in water distribution systems (Walski, 2001, 2005, 2111; Walski, Bowdler and Wu, 2005), Dr. Walski found in each case, “when a pump is selected to correctly match the system, the constant speed pump has a lower energy cost than the variable speed pump whether the storage is on the discharge side as in elevated water storage or on the suction side as in a wet well at a sewage pumping station”. So when are variable speed pumps more efficient? Dr. Walski states, “Variable speed pumping becomes more attractive in distribution systems that have no discharge side pumping and hence the pumps cannot be turned off. Instead, they must be run continuously.” Since variable speed pumps are not more efficient in waste water pump stations and maintenance costs for variable speed pumps can be significantly higher than those for constant speed pumps plus the initial purchase cost of variable speed pumps is known to be higher than constant speed pumps, there is a need for a new method to reduce energy and maintenance costs without significant costs of retrofitting existing pump stations.